1. Field of the Invention
The present invention relates to a particle beam therapy system that performs therapy by irradiating a particle beam onto a diseased site such as a tumor, and particularly to a three-dimensional-irradiation particle beam therapy system that irradiates a particle beam in accordance with the three-dimensional shape of a diseased site.
2. Description of the Related Art
In a treatment method based on a particle beam, a high-energy particle beam, such as a proton beam or a carbon beam accelerated up to 70% of the light velocity, is utilized. These high-energy particle beams have the following features when irradiated into a body. Firstly, almost all of irradiated particle beams stop at a position of a depth proportional to the particle beam energy raised to the 1.7th power. Secondly, the density (referred to as a dose) of energy, which is given to the path through which an irradiated particle beam passes until it stops in a body, becomes maximum at a position where the particle beam stops. A distinctive deep dose distribution curve formed along a path through which a particle beam passes is referred to as a Bragg curve. The position where the dose value becomes maximum is referred to as a Bragg peak.
A particle beam three-dimensional irradiation apparatus is contrived in such a way that, while it scans the Bragg peak position in accordance with the three-dimensional shape of a tumor and adjusts the peak dose at each scanning position, a predetermined three-dimensional dose distribution is formed in a tumor region, which is a target preliminarily determined by an imaging diagnosis. The scanning of the position where a particle beam stops includes scanning in transverse directions (X and Y directions) that are approximately perpendicular to the irradiation direction of a particle beam and scanning in a depth direction (Z direction) that is the irradiation direction of a particle beam. In the transverse-direction scanning, there exists a method of moving the position of a patient with respect to a particle beam and a method of moving the position of a particle beam by use of an electromagnet or the like; in general, the method utilizing an electromagnet is adopted. The method of scanning in the depth direction is performed only by changing the energy of a particle beam. As the method of changing energy, there exists a method of changing the energy of a particle beam by means of an accelerator and a method of inserting an energy attenuator into a path through which a particle beam passes and changing the attenuation amount of the attenuator. Such a method of changing the energy of a beam by varying the attenuation amount of an attenuator is disclosed, for example, in Patent Document 1 (Japanese Patent Application Laid-Open No. 2006-341010) or Patent Document 2 (Japanese Patent Application Laid-Open No. H10-199700).
The number of beam-energy changes in an actual particle beam therapy system depends on the size of a target, the king of a particle beam, and the maximum energy; as may be necessary, the beam energy is required to be changed approximately 100 times during irradiation. Accordingly, the speedup of beam-energy change leads to the reduction of a therapy time and the enhancement of the accuracy. In FIG. 2 of Patent Document 1, there is disclosed a beam energy changing apparatus that rapidly changes the energy of a beam. In this conventional technology, a beam that enters the beam energy changing apparatus is once deflected from the incident direction by use of two pairs, i.e., totally four deflection electromagnets; the particle beam that has reached a certain position is bent again to the opposite direction; the traveling direction of the particle beam is made to coincide with an orbit that is approximately in parallel with the extended line of the incident direction of the incident beam. The particle beam that has traveled a predetermined distance along the parallel orbit is bent in such a way as to return to the extended line of the incident direction of the particle beam. A range shifter (energy attenuator) having portions of different thicknesses is disposed in the parallel orbits; by changing parameters for the four electromagnets, the parallel orbits are made to correspond to the portions of different thicknesses of the range shifter so that the energy of an incident particle beam is changed. The particle beam, the energy of which has been changed, is transported to the particle beam irradiation apparatus.
According to the methods disclosed in FIG. 2 of Patent Document 1, the energy of a particle beam can extremely rapidly be changed compared with the method, disclosed in FIG. 6 of Patent Document 2, in which a range shifter is mechanically moved. However, in the case where the number of required beam-energy changes is large, the size of the range shifter becomes large and hence it is required to deflect a particle beam by a large angle; therefore, the size of the electromagnet becomes large.